High impact resistant ceramic composite

ABSTRACT

A ceramic material and a method of forming a ceramic material which possesses a high impact resistance. The material comprises: (a) a first continuous phase of β-SiC; and (b) a second phase of about 25-40 vol % TiB 2 . Al 2  O 3  is preferably used as a densification aid. The material is formed by hot-pressing the mixture at a temperature from greater than about 1800° C. to less than the transition temperature of β-SiC to α-SiC. The hot-pressing is performed at a pressure of about 2000 psi to about 4000 psi in an inert atmosphere for several hours and results in the formation of a two phase sintered ceramic composite material.

BACKGROUND OF THE INVENTION

The U.S. Government has rights to this invention pursuant to Contract No. DE-AC04-87AL4254 awarded by the Department of Energy (DOE).

1. Field of the Invention

This invention relates to high impact resistance ceramic materials. More particularly, this invention relates to a process for forming high density silicon carbide/titanium diboride composite materials by hot pressing blended powders with the addition of Al₂ O₃ as a sintering aid.

2. Description of the Related Art

There is a need for a lightweight ceramic material having a high impact resistance. An increase in the impact resistance of ceramic armor materials will allow less ceramic material to be used to attain a desired impact resistance. Less ceramic material means that a vehicle containing that material is lighter and will have greater fuel efficiency. In aircraft, the lighter weight also means higher payloads. In addition, ceramic armor has been found to be effective in body armor, including vests for the protection of ground troops and police.

Lightweight ceramic materials formed from ceramics such as silicon carbide, titanium diboride, aluminum nitride, and aluminum oxide mixed with phases of other metal oxides have been found to be desirable from the standpoint of weight, strength, impact resistance, high temperature durability and chemical inertness. However, there still exists a need for a ceramic material which possesses a higher impact resistance.

Sintered ceramics made from compositions containing titanium diboride in a matrix of silicon carbide have been found to have very desirable properties. Silicon carbide does not occur naturally. It is produced by a high temperature chemical reaction of silica and coke. The reaction is most economically performed in a resistance furnace with the voltage passing through the resistance core generating temperatures of about 2400° C. Another method of producing SiC is by carbothermic reduction.

Research that has been conducted on these compositions have been performed primarily on material that contains alpha silicon carbide (α-SiC). Alpha silicon carbide has either a hexagonal or rhombohedral unit cell. The cubic form of SiC is commonly referred to as beta silicon carbide (β-SiC). β-SiC has not been examined as extensively for armor applications because the SiC material that is used in forming the armor is processed at temperatures above 2000° C. If the SiC is formed or fired above the temperature at which β-SiC is transformed into α-SiC (2000° C.), the SiC product will be α-SiC.

McMurtry et al in an article entitled "Microstructure and Material Properties of SiC-TiB₂ Particulate Composites" in The Ceramic Bulletin, Feb., 1967, Vol. 66, No. 2, disclose the effect of a 16 vol % addition of TiB₂ to α-SiC on mechanical, thermal, and electrical behavior. The material was pressureless sintered in an inert atmosphere at temperatures greater than 2000° C. The TiB₂ particles in a SiC matrix were shown to have caused crack tip deflection which resulted in a higher fracture toughness than compared with single phase sintered SiC, the pressureless sintered particulate composite, α-SiC-TiB₂, exhibited up to a 90% increase in fracture toughness to 8.9 MPa and a 30% increase in flexural strength to 478 MPa at 20° C. with mechanical strength retention at temperatures less than 1200° C.

Seshadri et al in an article entitled "Fabrication and Mechanical Reliability of SiC-TiB₂ Composites" at the Third International Symposium, Ceramic Materials and Components for Engines, American Ceramic Society, Las Vegas, November 1988, disclose the enhanced mechanical and fracture properties of α-SiC-TiB₂ ceramic composites over monolithic α-SiC at temperatures in excess of 1000° C. The α-SiC-TiB₂ composite is a two-phase particulate composite material containing discrete TiB₂ particles ranging from 1-5 microns. The TiB₂ phase is uniformly distributed throughout the matrix, with very little or no bonding present. The α-SiC-TiB₂ composite was fabricated by pressureless sintering.

U.S. Pat. No. 4,327,186, issued to Murata et al, discloses a pressureless sintered silicon carbide-titanium diboride mixture and articles therefrom. The silicon carbide-titanium diboride material is useful in the fabrication of diesel engine precombustion chambers and honeycomb structures, such as those utilized in automobile emission control units. The patent teaches a material containing 5-20 wt % titanium diboride and 0.2 to about 3.0 wt. % of a densification aid. Boron-containing compounds, such as boron carbide, are preferred as a densification aid. The material has a 97.0% relative density. Murata et al teach that both α-SiC and β-SiC are suitable SiC phases for use in the Murata et al invention. However, since the materials in the examples are sintered at 2150° C. for one hour, one would expect that if β-SiC is used in the initial mixture, it would be transformed into α-SiC and there would not be any significant amounts of β-SiC present in the sintered product.

It would be advantageous, therefore, to provide a process whereby readily available and relatively inexpensive initial reactant materials can be formed into a sintered ceramic composite possessing a higher impact resistance than has been previously attained from ceramic materials.

The principal object of the present invention is to provide a high strength ceramic composite possessing an impact resistance that is higher than its individual components and higher than prior art compositions.

Another object of the present invention is to provide a process for producing a two phase ceramic material possessing superior impact resistance to projectile penetration.

These and other objects and advantages will be more fully understood and appreciated with reference to the following description.

SUMMARY OF THE INVENTION

The present invention relates to ceramic material and a method of forming a ceramic material which possesses a high impact resistance. The material of the present invention comprises: (a) a first continuous phase of about 60-75 vol % B-SiC; and (b) a second phase of about 25-40 vol % TiB₂. The first and second phases are at least semi-interlocking. Al₂ O₃ is preferably used as a densification aid.

In a preferred embodiment of the material of the present invention, the ceramic material is a hot pressed composite having a density of at least 99% of theoretical density, and preferably greater than about 99.9%. The hot pressed composite contains agglomerates of TiB₂ particles dispersed in a β-SiC matrix. The TiB₂ particles that are used in the composition preferably have an average grain size of about 5.0 microns and they form agglomerates of about 20.0 microns and larger during hot pressing. These TiB₂ agglomerates form a semi-continuous phase. Surprisingly, the hot pressed ceramic composite material has an impact resistance which is much higher than either of its major components and also higher than SiC/TiB₂ which is produced by pressureless sintering. The increase in the impact resistance of the hot pressed β-SiC/TiB₂ material is greater than one would expect from changing the sintering process step to hot pressing.

The preferred method of the present invention includes: (a) mixing TiB₂, β-SiC and Al₂ O₃ in an organic medium; (b) drying the mixture; and (c) hot-pressing the mixture at a temperature from greater than about 1800° C. to less than the transition temperature of β-SiC to α-SiC. The hot-pressing should be performed at a pressure of about 2000 psi to about 4000 psi in an inert atmosphere for several hours and results in the formation of a two phase ceramic composite material. The silicon carbide in the hot pressed material is present in its beta form.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features of the present invention will be further described or rendered obvious in the following related description of the preferred embodiment which is to be considered together with the accompanying drawings, wherein:

FIG. 1 is a photomicrograph showing the microstructure of the preferred ceramic material of the present invention.

FIG. 2 are photomicrographs showing the microstructure of the ballistic fracture surface of the preferred ceramic material of the present invention at different magnifications.

FIG. 3 is a bar graph comparing the ballistic performance of the material of the present invention with other high performance impact materials.

FIG. 4 is a Table showing selected properties of the preferred material of the present invention and prior art materials.

FIG. 5 is a photomicrograph showing the microstructure of a sintered prior art material, Hexaloy-ST.

DETAILED DESCRIPTION OF THE INVENTION

To produce a hot-pressed ceramic material having an improved impact resistance by the process of the present invention, initial reactants β-SiC and TiB₂ are provided in volume ratio such that the TiB₂ is from 25-40 volume percent of the resulting β-SiC and TiB₂ mixture. 0.5-3.0 wt. % Al₂ O₃ is blended into the mixture and it is hot pressed to form a β-SiC-TiB₂ composite material.

The silicon carbide component is a cubic or β-phase silicon carbide. The preferred particle size of the β-SiC powder is between about 0.3 microns and about 1.5 microns. The most preferred particle size of the β-SiC is in the submicron range and is between about 0.5 microns and 1.0 microns with an average particle size of 0.7 microns. The β-SiC material that is used in the present invention is formed by carbothermic reduction. This process produces a β-SiC material having particles in the submicron range and no comminution of the material is needed. A carbothermically produced β-SiC material having a particle size distribution which falls within the preferred particle size range is commercially available from H. C. Starck Inc. Alternatively, suitable silicon carbide material may be produced by grinding, ball milling or jet milling larger particles of silicon carbide and subsequently classifying or separating the desired component.

The titanium diboride starting component that has been found to be useful in the present invention has an average particle size between about 3.0 microns and about 8.0 microns. The most preferred average particle size for the titanium diboride component is about 5.0 microns. This titanium diboride material is commercially available from H. C. Starck Inc.

The Al₂ O₃ component is a densification aid that is used to assist in hot-pressing the initial β-SiC and TiB₂ materials. Preferably, the Al₂ O₃ is a high purity material and has an average particle size of less than 1.0 micron. As explained in greater detail below, it is mechanically mixed with the initial reactants β-SiC and TiB₂ prior to hot-pressing. The Al₂ O₃ used in the examples was obtained from Baikowski Inc.

The components are thoroughly mixed prior to hotpressing to obtain an intimate mixture. The means that are employed to mix the components are not considered to be critical to practicing the invention. However, since TiB₂ powders are highly reactive, nonaqueous blending and drying procedures are required for material processing. Excellent results have been obtained by ball mill, however other mixing methods familiar to one skilled in the art are also suitable.

The powders are wet milled under methanol using alumina media for 3 to 8 hours. After mixing, the slurry is transferred to drying trays and dried at a low temperature, such as 110° C., for example, to drive off alcohol. The material is then put through a 20 mesh screen and the resultant powder is free flowing.

The blend is then poured into a graphite die mold. After filling the molds, the temperature of the reactants is elevated by a suitable heating means to a temperature of at least 1800° C., preferably at least 1900° C. while applying a pressure of from about 2000 psi to about 4000 psi. The hot pressing is performed in an inert atmosphere such as helium or argon. It is believed that by maintaining the reactants at a given temperature of at least 1900° C. and at a pressure of at least 2000 psi for a sufficient period of time, initial reactants are compacted to a form a multi-phase β-SiC/TiB₂ composite material. During hot pressing, some of the TiB₂ particles agglomerate and form a semi-continuous phase in the continuous β-SiC matrix.

The period of time required to accomplish this reaction will vary with the temperature and pressure employed as well as the molar ratios of the components. Within a temperature range of from about 1900° C. to 2000° C., a heating time of 1-3 hours has been found to be sufficient to yield a two phase β-SiC/TiB₂ material in excess of 99% theoretical density. The upper limit on the temperature range that is used to hot press the material is the temperature at which β-SiC is transformed into α-SiC. This transition temperature is approximately 2025° C.

The following examples are offered to illustrate the process of the present invention.

EXAMPLE 1

SiC-TiB₂ ceramic particulate composite material possessing a high fracture toughness is prepared from a mixture of β-SiC and TiB₂ in a 70:30 volume percent ratio. A 1.5 wt. % addition of alumina is blended into the mixture as a densification aid. This mixture is then charged into a 5.0 gal Nalgene ball mill where it is uniformly mixed for 4 hours using one gallon of methanol.

After ball milling, the slurry is dried at 100° C. to drive off methanol. The material is then put through a 20 mesh screen and the resultant powder is free flowing.

The blend is then place into a tooling mold to form plates. After filling the molds, temperature of the reactants is elevated to a temperature of approximately 1950° C. while applying a pressure of about 2500 psi for 2 hours. The blend is compacted into a material which is essentially a two phase β-SiC/TiB₂ composite having a density in excess of 99% of theoretical density.

FIG. 1 is a photomicrograph (1000× magnification) of a polished and etched specimen of the hot pressed material of Example 1. The β-SiC/TiB₂ composite is a dense continuous β-SiC matrix reinforced with a semi-continuous titanium diboride phase. The light gray areas in FIG. 1 are TiB₂ and the dark gray is the continuous β-SiC phase. From the photomicrograph of FIG. 1, it can be seen that the TiB₂ material has formed into agglomerates. The TiB₂ agglomerates have a length in excess of 20 microns.

The some of the physical properties of the hot pressed material are measured and determined to be as follows:

    ______________________________________                                         Density (theoretical)                                                                              3.59 g/cm.sup.3 (99.9%)                                    Average Porosity    less than 1.0%                                             4 Point Bend        60.5 × 10.sup.3 psi                                  Elastic Modulus     75.3 × 10.sup.6 psi                                  Sonic Velocity      11750 M per sec.                                           Hardness            2768 Kg per mm.sup.2                                       Fracture Toughness  521 MPam.sup.0.5                                           ______________________________________                                    

The hot pressed material is then fabricated into armor test targets by placing them into 6061-T6 aluminum alloy picture frames. The impact resistance of the material is then ballistically tested using a medium caliber armor piercing projectile.

FIG. 2 contains photomicrographs showing the microstructure of the ballistic fracture surface of the β-SiC/TiB₂ material of Example 1 at 400× and 1000× magnification. It appears that the addition of TiB₂ agglomerates in the β-SiC matrix increased the crack path through the material during impact. The photomicrograph indicates that the crack propagated via a transgranular mode in the β-SiC matrix and via an interganular mode around the TiB₂ agglomerates. The agglomeration of the TiB₂ particles during hot pressing has thus increased the length of the crack.

FIG. 3 is a bar graph comparing the results of ballistic performance of the β-SiC/TiB₂ material of Example 1 with other high impact performance ceramic materials. The results of impact testing is recorded as mass efficiency (Em) which has been normalized to hot pressed TiB₂. High mass efficiency is a desirable criteria for armor applications. The ballistic performance of the β-SiC/TiB₂ material of Example 1 was found to be significantly better than any other materials tested. The performance of the hot pressed β-SiC-TiB₂ material was approximately a 20% improvement over the next best material tested, a hot pressed TiB₂. It is believed that the crack deflection caused by the TiB₂ agglomerates in the matrix are responsible, at least in part, for the superior impact resistance of the β-SiC/TiB₂ material.

EXAMPLE 2

The ballistic testing is performed on a sample of pressureless sintered SiC/TiB₂ that was obtained from Carborundum Corp. This material is commercially available and sold under the trademark Hexaloy-ST. The impact resistance of the pressureless sintered SiC/TiB₂ material is then ballistically tested using a medium caliber armor piercing projectile according to the procedure of Example 1. The comparative ballistic results are shown in FIG. 3. The performance of the hot pressed β-SiC/TiB₂ material is approximately 50% higher than the performance of the SiC-TiB₂ of Example 2.

FIG. 4 is a photomicrograph (1000× magnification) of a polished and etched specimen of the pressureless sintered SiC/TiB₂ composite material of Example 2. The light gray areas in FIG. 4 are TiB₂ and the dark gray is the continuous α-SiC phase. From the photomicrograph of FIG. 4, it can be seen that the TiB₂ material has not formed into agglomerates as large as those in the hot pressed β-SiC/TiB₂ material. The TiB₂ agglomerates have a length of less than 10 microns.

EXAMPLE 3

The ballistic testing procedure of Example 1 was repeated with 38 other materials including TiB₂, SiC, Al₂ O₃, A.B (Al₂ O₃ /B₄ C multiphase composite material), AlN, and an AlN/TiB₂ laminate. The 10 best ballistic materials were included in the bar graph of FIG. 3. Selected properties of these materials can also be seen in the Table of FIG. 5. The Table of FIG. 5 indicates the vendor from which each material was obtained.

The comparative ballistic results are shown in FIG. 3. The performance of the hot pressed materials all fall within plus or minus 20% of the mass efficiency of hot pressed TiB₂. The β-SiC/TiB₂ material of Example 1 is the only material that performed better than hot pressed TiB₂. The β-SiC/TiB₂ material performed approximately 20% better than the hot pressed TiB₂ and 25% better than hot pressed SiC.

While the inventor does not wish to be bound by any theory of why the hot pressed ceramic materials possess an improved impact resistance, it appears that the superior results obtained in the β-SiC/TiB₂ material of Example 1 is the result of the use of the beta form of SiC, the addition of a material having an elastic moduli higher than the β-SiC, the presence of a continuous phase of TiB₂ within the β-SiC matrix, and the use of Al₂ O₃ as a sintering aid.

It is contemplated that the β-SiC-TiB₂ composite of the present invention will also be useful as wear inserts and roller followers for engine components. Other applications include nozzles, seals, exhaust components and bearing guides that require high wear resistance and/or erosion resistance and armor applications with enhanced ballistic impact resistance.

Although the present invention has been described in terms of a TiB₂ powder, it is also contemplated that TiB₂ single crystal whiskers or platelets may also be used as a reactant material in practicing the invention. It is contemplated that whisker agglomeration and orientation during hot pressing may cause an increased impact resistance.

In addition, although the invention has been described in terms of a hot pressed two phase β-SiC-TiB₂ material, it is believed that this material is only one species in a genus of hot pressed multiphase materials in which the minor component is present in sufficient quantities so that chainlike agglomerates are formed and the minor component has an elastic modulus greater than the matrix component.

Furthermore, although the invention has been described in terms of a material that has been hot pressed using an Al₂ O₃ densification aid, it is believed that other densification aids may also be used in practicing the invention. Other suitable densification aids include free silicon and free carbon. However, doping β-SiC with boron has been known to accelerate the β to α transition. Therefore, it is believed that boron containing material should not be used as a densification aid.

While the invention has been described in terms of preferred embodiments, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative. The present invention is indicated by the broad general meaning of the terms in which the appended claims are expressed. 

What is claimed is:
 1. A method of forming a ceramic material having a high impact resistance, said method comprising the steps of:(a) forming a mixture of 60-75 vol. % β-SiC, 25-40 vol. % TiB₂ and 0.5-3.0 wt. % of Al₂ O₃ as a densification aid; and (b) hot pressing said mixture at a temperature less than the transition temperature of β-SiC to α-SiC to form a mixed phase β-SiC/TiB₂ material.
 2. The method according to claim 1 wherein said step of forming a mixture β-SiC and TiB₂ includes:mixing β-SiC powder having an average particle size between about 0.3 microns and about 1.5 microns.
 3. The method according to claim 1 wherein said step of forming a mixture of β-SiC and TiB₂ includes:mixing TiB₂ powder having an average particle size between about 3.0 microns and about 8.0 microns.
 4. The method according to claim 1 wherein said step of forming a mixture of β-SiC and TiB₂ includes:wet milling said mixture in a nonaqueous solution; drying said mixture in air; and screening said mixture to minus 20 mesh.
 5. The method according to claim 1 wherein said step of hot pressing said mixture includes heating said unreacted mixture to a temperature from about 1880° C. to 2000° C.
 6. The method according to claim 1 wherein said step of hot pressing said unreacted mixture includes maintaining pressure of from about 2000 to about 4000 psi on said mixture.
 7. A hot pressed ceramic material having an improved impact resistance, the material comprising:(a) 60-75 vol. % of a continuous SiC matrix which is β-SiC; (b) 25-40 vol. % of a second TiB₂ phase; and (c) 0.5 to 3.0 wt. % Al₂ O₃.
 8. The material of claim 7 in which said TiB₂ phase contains agglomerates of at least 20 microns in length.
 9. The material of claim 7 in which said TiB₂ phase is semi-continuous.
 10. A ceramic composite armor material comprising:(a) a continuous matrix of 60-75 vol. % β-SiC; (b) 25-40 vol. % TiB₂ agglomerates; and (c) 0.5 to 3.0 wt. % Al₂ O₃.
 11. The material of claim 10 in which said TiB₂ agglomerates are at least 20 microns in length.
 12. The material of claim 10 in which said TiB₂ agglomerates form a semi-continuous phase.
 13. A method of forming a ceramic armor material, said method comprising the steps of:(a) forming a mixture of 60-75 vol. % β-SiC, 25-40 vol. % TiB₂ and 0.5 to 3.0 wt. % of Al₂ O₃ ; and (b) hot pressing said mixture at a temperature less than the transition temperature of β-SiC to α-SiC to form a mixed phase β-SiC/TiB₂ material.
 14. The method according to claim 13 wherein said step of forming a mixture β-SiC and TiB₂ includes:mixing β-SiC powder having an average particle size between about 0.3 microns and about 1.5 microns and TiB₂ powder having an average particle size between about 3.0 microns and about 8.0 microns. 